Answer:
The shrinking of the star due to gravitational forces internal to the core is counterbalanced by the pressure of the photons seeking to get out.
Inside stars nuclear fission happens that is like hundreds of nuclear bombs going off. That creates pressure that presses out to keep the star at a constant size. Gravity presses in, explosions press out.
(Novas occur when these forces become out of balance and the core collapses, and the star blows off the outer shells of gas.)
Step-by-step explanation: